1. Field of Invention
The present invention relates to a photolithographic process. More particularly, the present invention relates to a thermal flow photolithographic process.
2. Description of Related Art
The fabrication of semiconductor can be separated into four major modules including diffusion, etching, thin film and photolithography. Photolithographic process includes the transfer of a pattern from a mask to a silicon chip so that the silicon chip can be etched in an etching module or dopants can be implanted in a thin film module. Hence, precision of pattern transfer in the photolithographic process will directly affect the final quality of the semiconductor circuits.
As the level of integration of integrated circuits continues to increase, the line pitch of bit lines, word lines, doped regions and capacitors is reduced correspondingly. Therefore, resolution of stepping machines must be increase to reproduce finer patterns. However, an increase in the resolution of a stepping machine will result in a loss in depth of focus. On the contrary, increasing depth of focus to obtain a better pattern transfer accuracy will result in a lower resolution. Aside from increasing the resolution of a stepping machine, better pattern transfer results can also be obtained by performing a thermal flow process to reduce line width of pattern on the photoresist layer.
To reduce pattern line width on a photoresist layer by thermal flow method, a photomask pattern is first transferred to a photoresist layer. A soft baking process is carried out and then the photoresist layer is developed. The photoresist layer is heated to a temperature above the glass transition temperature (Tg) on a hot plate. Ultimately, the photoresist layer will begin to flow and reduce the line width of the pattern.
Nevertheless, thermal flow is very temperature sensitive. In other words, minor temperature variation may result in large dimensional change in the pattern ultimately. Hence, process window for thermal flow is quite narrow (between 1xc2x0 C. to 2xc2x0 C.). In general, a hot plate is accurate to within 0.4xc2x0 C. to 0.5xc2x0 C. Thus, if a uniform line width is required in each silicon wafer after thermal flow, temperature of the hot plate must be precisely controlled. However, the demand for such a high precision increases not only the difficulties in fabricating the semiconductor devices, but also increases the cost of production.
Accordingly, one object of the present invention is to provide a thermal flow photolithographic process. A thermal flow photoresist is provided. A cross-linking agent is added to the thermal flow photoresist to form a high-temperature cross-linking photoresist material. A substrate having an insulation layer thereon is provided. The high-temperature cross-linking photoresist is deposited over the insulation layer. The cross-linked photoresist layer on the insulation layer is exposed to light, chemically developed and finally heated to cause a thermal flow.
Since the high-temperature cross-linking photoresist contains polymers with cross-linking functional groups, glass transition temperature of the photoresist is increased due to the cross-linking of polymers. Ultimately, process window for thermal flow is widened.
In addition, besides increasing glass transition temperature, the cross-linking of polymers also strengthens internal bonding strength of the photoresist layer. Hence, the photoresist layer is less vulnerable to attack by etching agents. Consequently, a higher etching selectivity ratio between the photoresist layer and the insulation layer is obtained.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.